The present invention was developed during ongoing research and developmental efforts by the assignee to improve performance of adaptive control systems. An example of an active acoustic control system developed by the assignee which is capable of attenuating non-periodic acoustic disturbances is disclosed in U.S. Pat. No. 5,621,803 entitled "Active Attenuation System With On-Line Modeling of Feedback Path", by Trevor A. Laak, issued on Apr. 15, 1997, assigned to the assignee of the present application, incorporated by reference herein. In many active control applications, cancellation is required only at discrete frequencies where tonal disturbances exist. An example of an adaptive tonal control systems and methods developed by the assignee is disclosed in copending U.S. patent application Ser. No. 08/369,925 entitled "Adaptive Tonal Control System With Constrained Output and Adaptation", by Steven R. Popovich, filed on Jan. 6, 1995, now U.S. Pat. No. 5,633,795, issued on May 27, 1997 which is incorporated by reference herein.
Problems can sometimes develop in adaptive control systems when the controller attempts to drive one or more of the actuators (i.e., loudspeakers in a sound attenuation system) beyond physically sustainable limits. For low or medium actuator output, the transfer function for actuators is characteristically linear. However, when actuator output becomes high, the actuator transfer function becomes non-linear and the system can become unstable and/or physical components of the system can be damaged. It is therefore desirable to constrain controller output so that the maximum output of each actuator is limited within the linear range of each individual actuator. One way of constraining controller output involves the use of leakage methods, but leakage methods can compromise overall system performance when used for the purpose of limiting output power. Examples of power limiting using leakage techniques include the system disclosed in copending patent application Ser. No. 08/553,186 entitled "Frequency Selective Active Adaptive Control System", by Shawn K. Steenhagen et al., filed on Nov. 7, 1995, assigned to the assignee of the present application, now U.S. Pat. No. 5,710,822; and U.S. Pat. No. 5,627,896 entitled "Active Control of Noise and Vibration", by Steve C. Southward et al., issued on May 6, 1997.
In many active control applications, it is necessary to use multiple inputs and multiple outputs to attain effective control. The use of high numbers of sensors and actuators along with sophisticated adaptation schemes can stretch computational requirements beyond those practical. It is therefore not only important that adaptation converge reliably to an adequate solution, but also that adaptation occurs efficiently within realistic signal processing requirements.
The filtered-X algorithm is an effective means for controlling disturbances at multiple locations when there are a relatively small number of sensors and actuators. However, as the number of actuators and error signals becomes large, convergence rates tend to slow. Normalizing adaptation to provide more direct convergence improves tracking in tonal systems, and also benefits performance in feedforward systems cancelling random disturbances.
It is desirable to provide normalized adaptation for quick convergence while at the same time limiting individual actuator output so that the effectiveness of each individual actuator is maximized, all without exceeding reasonable signal processing resources provided by conventional digital signal processors used for active acoustic attenuation. It is also important for the limiting of the actuator outputs to be performed in a manner which is compatible with the normalization of the adaptation so that these functions may be performed simultaneously.